SEABIRD
OBSERVATIONS
OFF WESTERN
MEXICO
STEVE N. G. HOWELL, PointReyesBird Observatory,
4990 ShorelineHighway,
StinsonBeach, California94970
STEVEN J. ENGEL, 1324 SecondStreet, Bismarck,North Dakota 58501
Southern Mexico's offshore Pacific avifauna has been rather little studied.
Murphy (1958) and Jehl (1974) reportedobservations
from cruisesthat
passedthroughMexicanwatersin Novemberand December1956 and in
April 19.73,respectively.
Pitman(1986) mappedthe relativeabundanceof
57 speciesof seabirdsin the easterntropicalPacificon the basison 4333
hoursof observationbetween 1974 and 1984; only a smallnumberof
these4333 hours(81 noon positions,55 of whichwere off Baja California), however,pertainto Mexicanwaters(i.e., within 200 nauticalmilesof
Mexican territow), and 74% of the 81 noon positionswere between
Octoberand March (R. L. Pitmanpers. comm.).Pitman'satlasprovidesan
excellentlarge-scale
picturebutlacksdataon seasonalstatusof species,and
the scale employeddoes not enable one to interpret local distributions.
Other recordsof seabirdsoff western Mexico are widely scatteredand
mostlyderivefrom nearshoreland-based
tripsof a dayor less(e.g., Binford
1970, 1989).
The Middle AmericanTrench runs from the vicinityof the IslasTres
Marias,Mexico,to the CocosRidge,southof CostaRica. The trenchlies
some 55-110 km (mean distance 75 km) offshore between Jalisco and
Guerreroand is 15-50 km (mostly20-30 km) wide. The trenchis at least
3600 m deep, mostly4300-4650 m deep from centralJaliscosouth,and
increasesto 5000-5200 m deep off centralGuerrero;submarinemountains in the trench off Colima (Manzanillo)and Guerrero (Zihuatanejo)
reduce depths to 3600 m. On either side of the trench waters quickly
shallowto 2700-3200 m, and inshorethe 1000-fathom (1800-m) contour
line lies20-55 km (mostly35-55 km) off the coast.
From 28 April to 6 May 1992 we observedseabirdsoff westernMexico
out to 100 km from shore,that is, from just offshoreof to well inshoreof
the Middle AmericanTrench, betweenthe vicinityof Cabo Corrientes,
Jalisco,and Acapulco, Guerrero. Here we describethe birds observed
duringour visit.
ITINERARY
AND METHODS
We beganobservations
at dawn on 28 April at 19ø51' N, 106ø28' W,
about 110 km west-southwest
of Cabo Corrientesand justoffshoreof the
northernend of the trench. Figure 1 showsthe routesof our daylight
transits off western Mexico relative to the Middle American Trench. Other
than 29 April and 4 May, when we madestopsfor supplies,dailytransits
were 85-145 km, with variationdue to time spentobservingfeedingflocks
and the effectof currentson our cruisingspeed.On 3 May we circledLas
RocasPotosi(17ø32' N, 101ø31' W), 13 km southeastof Zihuatanejo,and
on 4 May we checkedvariousislets 10-18 km west of Zihuatanejo
(includingLas IslasBlancasand Isla Grande).
Western Birds 24:167-181, 1993
167
SEABIRD
OBSERVATIONS
We madeobservations
with 8 x 30 binoculars
throughoutthe day from
the deckof the 14.5-m ketchEnchantress.Our eye levelwas about3 m
abovesea level,givinga visiblehorizonof 5.5-7 km. For 30 continuous
minutesof everyhour of transitwe countedall birdswithin 200 m of the
vesselwithina 90øsector(aheadto eitherportor starboard,
depending
on
light, wind, and/or sailconditions).The radiusof 200 m was chosensince
beyondthat distance,in windsof more than 10-15 km/h, we often were
unableto identifyto speciessmallerbirdssuchas storm-petrels
or phalaropes.Our speedoverthe groundaveraged11 km/h andwasrarelybelow
9 or above 13 km/h.
We frequentlychangedcourseto observefeedingflockswithin5 km of
our coursemore closely,and we notedthe compositionand behaviorof
speciesin each flock (from 5 to 45 minuteswere spentat each feeding
flock).We discontinued
any 30-minuteobservation
periodinterruptedby
suchcoursechangessinceit no longerrepresented
a randomtransit.
One or bothof uswereon deckduringall daylighthoursandmaintained
a nonstandardized
watchfor seabirds(i.e., anythingwe couldseefrom the
vesselregardless
of distanceand direction)when not making30-minute
censuses
or observing
feedingflocks.
We took the surfacewater temperatureat the startof each 30-minute
observation
periodandat the locationof eachfeedingflock.Positions
were
determinedby meansof a MagellanGlobalPositioningSystem;depths
wereinterpolatedto the nearest50 m from DefenseMappingAgencychart
numbers21017 and 21020. The warmestwaterswereconsistently
those
inshoreof the trench(in depthsof 900-3000 m), which averaged0.51.0øCwarmerthanwatersfromthe inshoreedgeof the trenchto offshore
of the trench.Monthlymeansea-surface
temperatures
for thesewatersare
1.0-2.5øC coolerin April and0.5-2.0øC coolerin May thanthe temperatureswe recorded(Robinson1976). The relativelywarm watersof our
cruiseprobablyreflecta well-developed
E1Nifio-SouthernOscillation.
Windsweregenerallylight(1-10 km/h), althoughfor briefperiodson 29
and 30 April they increasedto 20-25 km/h. Wind directionwas mostly
from southto westoff Jalisco,Colima,and Michoacan,and mostlynorthwest to west off Guerrero.
RESULTS
AND DISCUSSION
Table1 presentsthe resultsof our standardized
dailycensuses
asbirdsper
hour (totalindividuals
recordedper day duringcensusperiodsdividedby
total daily hoursof census).The followingspeciesaccountsdescribethe
abundanceand distribution
of the 30 speciesof seabirdsrecordedmore
than 3 km from shoreand otherspeciesof note. Commoncoastalspecies
suchas the Brown Pelican(Pelecanusoccidentalis)and LaughingGull
(Larus atticilia) are thus omittedfrom the accounts.
Whiletermssuchas "rare,""common,"etc., willalwaysbe subjective,
we
considerthem useful;data for each speciesindicatethe basisfor our
assessments
of abundance.
The numbersgivenin parentheses
aftereach
species'name indicatethe total numberobserved(includingstandardized
andnonstandardized
observations
andfeedingflocks,butnot individuals
at
168
SEABIRD
OBSERVATIONS
inshore
rocksandislets),
maximum
number
recorded
perday,andnumber
of daysrecorded.An asteriskafter the daily maximumindicatesthat the
hightotalwasrecordedon two days.Water temperaturerangesare givenat
the end of each speciesaccount.
SpeciesAccounts
LaysanAlbatross,Diomedea immutabilis (1; 1/1). Rare. We saw one on 28 Apr
associating
with a feedingflock over the northernend of the trench;29.4øC. This
specieshascolonizedislandsoff westernMexicoin recentyears(Howelland Webb
1992); Isla San Benedictois the breedingsitenearestour observation.
Pink-lootedShearwater,Puffinus creatopus (481; 160/8). Common to fairly
common; lesscommon in the southernhalf of our studyarea (Table 1). We recorded
Pink-looted
Shearwaters to within 20 km of shore but most were more than 25 km
offshoreover watersmore than 1000 m deep. This speciesoften was a common
memberof feedingflocksovermidshoreand offshorewaters.25.6-29.4øC.
Wedge-tailedShearwater,P. pacificus(124; 75/5). Fairlycommonto uncommon
in southernwaterswarmer than 27.5øC, with only one seen north of Michoacan.
Wedge-tailedShearwatersoccurredto within3.5 km of shorebut mostwere 20-75
22•
Figure1. Routesof seabirdobservations
off westernMexico(solidlineswitharrows),
28 April-6 May 1992. Dashedline indicatesbottomof the MiddleAmericanTrench.
A, Islas Tres Marias; B, Isla Isabel; C, Islas Tres Marietas; Ac, Acapulco; Mz,
Manzanillo;Zh, Zihuatanejo.
169
Table 1 SeabirdsObserved(to Nearest0.1) per Hour During30-Minute
Countsof 90øSectorsoff WesternMexico,28 April-6 May 1992
28
29
30
1
2
3
4
5
6
Apr
Apr
Apr
May
May
May
May
May
May
3.0
2.0
0.2
0.8
0.6
1.1
--
1.7
2.2
*a
__
--
-0.2
0.3
+
0.9
+
1.4
0.6
---
0.2
0.5
-0.2
--
--
--
0.5
+
0.6
0.3
0.5
+
--
0.6
2.9
--
0.5
8.7
--
--
--
--
+
2.0
1.0
--
0.2
2.9
0.5
+
--
--
0.3
--
0.8
9.1
Storm-Petrel
Black
Storm-Petrel
Least
Storm-Petrel
Red-billed
4.8
0.5
0.4
1.3
3.4
1.4
2.0
0.2
5.5
6.1
5.0
1.6
1.3
2.8
4.0
6.0
2.0
3.5
--
1.0
--
--
1.5
0.3
11.0
*
0.5
Tropicbird
MaskedBooby
BrownBooby
Red-lootedBooby
Magnificent
Frigatebird
--5.6
--
+
-2.0
--
--
+
0.2
0.2
6.2
0.7
0.5
8.0
2.0
0.3
3.1
+
-19.7
+
+
+
0.2
*
+
--
Phalarope
3.8
Red Phalarope
1.3
Phalaropesp.
1.3
PomarineJaeger 0.2
ParasiticJaeger 0.6
Long-tailed
Jaeger -Jaegersp.
0.2
1.0
2.5
1.0
1.5
-*
--
0.4
0.2
0.2
0.4
1.0
0.7
4.0
+
5.2
6.2
1.9
0.3
6.6
2.0
0.9
0.3
+
+
+
--
+
*
*
Sabine's Gull
0.2
1.0
--
0.5
+
*
+
1.0
Arctic Tern
Least Tern
Bridled Tern
Black Tern
0.3
--0.8
+
*
+
6.0
BrownNoddy
--
Hours of
observation
Sea surface
Pink-looted
Shearwater
Wedge-tailed
Shearwater
SootyShearwater +
Christmas
Shearwater
Townsend's
Shearwater
Audubon's
Shearwater
Leach's
Storm-Petrel
Galapagos
*
5.0
--
0.7
0.2
12.5
+
ß
*
-2.0
3.0
+
1.2
1.5
3.7
0.3
0.7
5.6
6.2
0.5
,
*
0.9
+
+
*
0.9
1.1
+
+
+
ß
,
+
--
-0.2
0.4
--
+
0.4
Red-necked
Common Tern
Arctic Tern
+
1.0
+
--
+
1.5 10.8
--
-4.0
--
13.1
1.0
3.1
24.0
0.3
--
-0.4
--
6.25
2.0
5.5
4.0
3.25
3.5
1.0
6.0
5.5
25.628.5
27.527.6
27.028.4
27.729.4
28.•
29.4
28.•
29.3
28.4
27.730.2
27.228.8
Common/
temperature
(rangein øC)
--
+
6.5
*
+
a+, seenoutsidecensusperiods;* seenoutsidecensusperiodsand only with feedingflocks.
170
SEABIRD
OBSERVATIONS
km offshoreover waters900-5000 m deep. We found most birdswith feeding
flocks,andexceptfor two or threedark-morphbirdson 2 May we sawonlythe light
morph,agreeingwiththe resultsof Pitman(1986). 27.7-29.5øC.
SootyShearwater,
P. griseus(29; 5*/7). Uncommon,seenmostlyas singlebirds
flyingnorthwest,but a few birdsassociated
looselywith feedingflocks.Sooty
Shearwaters
occurred18-95 km (mostlybeyond35 km)offshoreoverwatersgreater
than 1000 m deep. 27-29.5øC.
Christmas Shearwater, P. nativitatis (47; 27/4). Fairly common over waters
warmerthan 28.0øC and 2700-5000 m deep in a belt 35-55 km offshoreof
Guerrero;uncommonoff Michoacan,wherewe sawthe northernmostbirdat 17o53'
N, 103o41' W. We foundmost ChristmasShearwaterswith feedingflocks.27.729.4øC.
The A.O.U. (1983) considered
thisspecies"accidental
at seabetweenClipperton
Islandandthemainlandof Mexico,"presumably
on thebasisof an undatedspecimen
collected at 12o05' N, 107 ø W (Loomis 1918), about 315 km northeast of
Clipperton.Pitman(1986), however,reportedseeingthis speciesat ratesof 0.752.0 per hour off westernMexico (from Michoacanto Oaxaca)betweenJune and
September;he collecteda specimen(LosAngelesCountyMuseum)on 2 June 1982
at 15o20' N, 99o22' W, about 180 km south-southeast
of Acapulco.In addition,D.
G. Ainleyand L. B. Spear(pers.comm.)observedfour ChristmasShearwaters102-
192 km offshorefromGuatemalato Oaxaca,Mexico,on 9 and 10 April 1992. This
species
appearsto bea fairlycommonvisitor,at leastduringsomeyearsfromAprilto
September,to watersoff southernMexico.
Townsend'sShearwater,P. auricularis(404; 133/7). Fairlycommon,occurring
25-95 km offshoreover waters 1000-4500 m deep. We detectedmostbirdsin
feedingflocksor as raftinggroupsof up to 45 birds.The morningof 28 April we
notedonesand twossteadilyflyingeast,i.e., on a directroute from the species'
nestinggroundson IslaSocorroto coastalwatersoff Jalisco.The plumageof mostof
the birdswe sawin the feedingflockswasworn and faded,and severalbirdswere
moltingflightfeathers;we notedonly 15-20 birds(including
thoseon 28 April
mentionedabove)thatappearedto be in fairlyfreshplumage.25.6-29.4øC.
Jehl (1982) concludedthat youngTownsend'sShearwatersfledgebetweenlate
May and late July. Thus, while someof the birdswe saw may have been failed
breeders,andothersbreedingadultsrangingto feedinggroundsoff westernMexico,
the majorityprobablywere immaturesandnonbreeders.
Jehl(1982), in plottingthis
species'seasonaldistribution,
citedonlyone recordfromwatersoff westernMexico
fromApril to June,andsawnoneoff westernMexicoin earlyApril 1973 or April
1976. Immatureand nonbreeding
Townsend's
Shearwaters
may arriveregularlyoff
southwesternMexico in late April.
Townsend'Shearwaters
associated
withAudubon'sShearwaters
at feedingflocks,
enablingdirectcomparison
of thesetwo species.Whilemostguidesemphasizethe
whitepatcheson the flanksof Townsend's,
mostbirdswe sawweresittingon the
waterandmadeonlyshortfeedingflights,sothe flankpatchesoftenwerenot visible.
In addition,many Townsend'swere very worn, faded,and brownishabove,and the
white flank patchesoften were inconspicuous.
The featureswe foundusefulin
separatingthesetwo speciesapplyonlyto nominateTownsend's
andthe sympatric
subspecies
of Audubon's
Shearwater
(P.I. subalaris,
nestingin the Galapagos).
Townsend'sis largerand longerwingedthan Audubon's,and thiswas obviousin
directcomparison.Althoughthe NationalGeographicSociety(1987) and Naveen
(1983) mentionedthe longtail of Audubon's,
we did not noticethis,and Murphy
(1936) pointedout that "the tail in the Galapagosrace is both relativelyand
absolutely
shorter"thanin Atlanticpopulations.
The quickerwingbeats
of Audubon's
usuallyidentifiedit in prolongedflight,but in feedingflocksflightmannerwasnot
171
SEABIRD
OBSERVATIONS
veryusefulsincebothspecies
mostlymadeshortrapidflights.We alsowereunableto
discernon birdsin feedingflocksreporteddifferencesin underwingpattern (e.g.,
Harrison1983, 1987), althoughthispatternmay be seenbrieflywhen birdsflush
from the water (Figure2).
Whenthe birdsweresittingon thewaterthe moststrikingdifferencewasthe white
sidesof Townsend's,suggesting
a TuftedDuck (Aythyafuligula),versusthe mostly
darksidesof Audubon's(Figure3). This differencemay be due in part to a tendency
of Audubon'sto holditswingsmoredrooped,since41 of 100 specimens
of P. I.
subalarisat the CaliforniaAcademyof Scienceshavecleanwhiteflanks;the other
59 havedistinctdarkstreakingon their flanks.
Faceand neck patternwas usefulin flightand at rest:Audubon'shasa clean-cut
dark cap that extendsto just belowthe eyesand may showindistinctwhite eyecrescents:Townsend'shas a more extensive and not clean-cutblackishcap that
extendswellbelowthe eyesand overthe auriculars
(Figure3). Townsend'shasa dark
half collar on the sidesof the lower neck; Audubon'shas a cleaner-cutand smaller
patch(Figure3). Atlanticpopulations
of Audubon's,
however,
havea darkhalfcollar
similarto Townsend's(Murphy 1936, Howell pers. ohs.).
The feet of Audubon'sappearedduskyflesh,thoseof Townsend'sfleshpink, not
black (contra Harrison 1983, Ridgely and Gwynne 1989). This is evident in
photographsof Townsend'staken by Howell (unpubl.)and Jehl (1982). Six specimensof Townsead'sat the CaliforniaAcademyof Scienceshaveobviouslypale legs
and feet with blackishonly on the outersideof the outertwo toes.as did two live
Newell'sShearwaters[P. (a.) newelli]examinednear Hawaii (Howelland L. B. Spear
fieldnotes).Thus the feet of Townsend's(andNewell's)appear mostlyfleshat sea,
perhapsbrighterandpinkerthanthe feet of Audubon's.
Figure2. Townsend's
Shearwateroff Jalisco,28 April 1992. Note extensivedusky
smudging
on sidesof faceandneck,relatively
fadedandbrownish
upperwings.
white
flankpatch,and extensivelycleanwhite underwing.
Photo by Steve N. G. Howell
172
SEABIRD OBSERVATIONS
Becauseof plumagewearandvariablelightingconditions
we foundthe colorof the
upperparts(usuallyconsideredblackishon Townsend'sand dark brown on
Audubon's)to be of littleusein fieldidentification.
Audubon'sShearwater,P. lherrninieri(126; 95/4). Fairlycommon20-35 km off
Guerrerooverwaters500-1800 m deep;we alsosawfivebirds20-45 km offJalisco
overwaters500-1000 m deep,northto 19ø51' N, 105ø37' W. Our lackof records
betweenGuerreroand Jaliscoprobablyreflectsour lack of observations
in these
waters. 27.6-29.5øC.
The A.O.U. (1983) reported Audubon'sShearwater as ranging north in the
easternPacificto Oaxaca,and Pitman (1986) reportedup to 2-10 birdsper hour
north to waters off Colima. Our reports off Jaliscorepresentthe northernmost
recordsof Audubon'sShearwaterin the region.
Leach'sStorm-Petrel,Oceanodromaleucorhoa(125; 75/6). CommonoffJalisco
in waterscoolerthan 27.5øC; uncommonto rare southfrom therein waterswarmer
than 27.5øC, wherewe sawonly 14 birds.Of the 13 dark-morphbirdswe saw,60%
Figure3. Comparisonof Audubon'sShearwater,Puffinus Iherminierisubalaris
(upper)andTownsend's
Shearwater,P. a. auricularis(loweft.
Sketch by Steve N. G. Howell
173
SEABIRD OBSERVATIONS
were over waterswarmerthan 27.5øC, while 96% of light-morphbirdswere over
waters cooler than 27.5øC. All Leach's Storm-Petrels were at least 45 km offshore
over waters2500-5000 m deep. 25.6-30.2øC.
AlthoughLeach'swas the commoneststorm-petrelon 6 May both immediately
beforeand aftera feedingflockthat included50+ Galapagosand 90+ Blackstorm-
petrels,onlythreeLeach'sassociated
withthisflock.
Galapagos(Wedge-rumped)
Storm-Petrel,O. tethys (321; 130/9). Commonto
fairlycommonthroughout
the cruise.We recordedit to within9 km of shore,but
most were at least 25 km offshore over waters 1000-5000 m deep. Notable
concentrations
were 130 birdsraftingwith 20 BlackStorm-Petrelsat 17ø03' N,
101ø26' W on 3 May, and 50+ raftingand feedingover BottlenoseDolphins
(Tursiopstruncatus)on 6 May.In 45 minutes,tenbirdscameto a slickof codliveroil
laidat 19o42' N, 105ø35' W, 6 May. 25.6-29.5øC.
Black Storm-Petrel, O. melania (487; 166/9). Common to fairly common
throughout,occurring2-95 km fromshoreoverwaters75-5000 m deep.Up to 12
birdsoften followedthe boat and scavengedgalleyscrapsafter meals.A notable
concentration
was90+ birdsraftingandfeedingoverBottlenose
Dolphinson 6 May.
In 45 minutes,26 birdscame to the same slickof cod liver oil that attractedthe
GalapagosStorm-Petrels
on 6 May. 25.6-30.2øC.
Least Storm-Petrel,O. microsoma(134; 46/6). Common within 35 km (and
mostlywithin 25 km) of shoreover waterslessthan 1000 m deep. The species'
abundance
in thiszoneismaskedin Table1 bytheinclusion
of hoursfartheroffshore
when none was seen.The slickof cod liver oil laid on 6 May attractedfive. 27.728.8øC.
Red-billedTropicbird,Phaethonaethereus(6; 2/5). Uncommonto rare. Evenon
29 Aprilwhenwe passedwithina few kilometers
of PefiaBianca,a largecolonyof
Red-billed
Tropicbirds
18 km westof Manzanillo,we sawonlytwo singletropicbirds.
None associated
with feedingflocks,and we notedonly one in our censusperiods.
Apparentlythisspeciesdisperses
widelyandis mostlysolitaryat sea;Au andPitman
(1986) recordedonlyonetropicbird
in 134 feedingflocksin thenortheastern
tropical
Pacific. 27.4-28.9øC.
Pefia Biancaappearsto be a previously
unreportedcolony,althoughit hasbeen
knownto localfishermenfor manyyears.HowellandS. Webbvisitedthe rockon 1
January1987 andobserved
900-1000 tropicbirds
withmanypairsengagedin noisy
courtshipflightsand some birds enteringcrevices.Subsequentobservations
by
Howell are of 50+ birdson 16 March 1989, 100+ on 15 March 1990, and 80-100
on 13 March 1992; on eachof thesedatesHowellsawsomecourtshipflightsand
severalbirdsenteringand leavingcrevices.
At Las RocasPotosi,Guerrero,we sawfour or five pairsof Red-billedTropicbirds
engaged
in screaming
courtship
flightsandsawonebirdentera crevicein therocks.
The A.O.U. (1983) recordedbreedingby Red-billedTropicbirdsalong Mexico's
Pacificcoastsouthonly to Las IslasTres Marias,Nayarit,but clearlythe species'
breedingrangeis more extensive.
MaskedBooby,Sula dactylatra(83; 40/4). Fairlycommon45-100 km offshore
fromMichoacansouth,overwatersdeeperthan2500 m. Mostwereassociated
with
feedingflocksor attracted
to theboat.All 70 adultsandsubadults
we sawclearlyhad
the brightyellow-orange
to pinkish-orange
billscharacteristic
of S. d. grantiof the
Galapagos,
suggesting
theyoriginated
there,althoughthissubspecies
alsonestsin
smallnumberson basIslasRevillagigedo
(HowellandWebb 1990). 27-28.2øC.
Blue-lootedBooby,S. nebouxii.We sawoneat Zihuatanejoon 3 May andtwo or
three at LaslslasBlancasand 11 or 12 at IslaGrande(18 km westof Zihuatanejo)on
174
SEABIRD OBSERVATIONS
4 May. This speciesis notablysedentaryaboutits nestingareas(southin Mexico to
Las IslasTres Marietas)and rarelywanderssouthto watersoff Colima (Howellpers.
obs.).We are aware of no previousrecordsfrom Guerrero,althoughthere is one
hypotheticalrecordfrom Oaxaca(Binford1989).
BrownBooby,S. leucogaster(5555; 1480'/9). Commonto fairlycommonout to
100 km from mainland.The two high countsreflect1310 birds(99% adult)heading
offshorefrom the largecolonyat Pefia Bianca,Colima,between0700 and 0730 on
29 April, and probablya largecomponent(1000+ birds)of the colonyat Las Rocas
Potosi, Guerrero (100-120 km distant), on 1 May. The Brown Booby was a
commonmemberof feedingflocksanda conspicuous
indicatorof themat distances
up to 5 km. Adultsconstituted60-95% of birdsseen on all daysbut 2 May; most
were commutingto and from the mainlandbecauseusuallywe saw few or none at
dawn when we often saw the more pelagicMaskedand Red-lootedboobies.On 1
and 2 May, however, we saw numerous Brown Boobies at dawn, 75-85 km
offshore,and on 2 May 90% of the 175 Brown Boobiesseenwere juvenilesand
immatures. 25.6-30.2øC.
At Las Rocas Potosi we counted 6000-6500 birds, includingmany recently
fledgedjuveniles,on or aroundthe rocks,1430-1510 on 3 May. The A.O.U. (1983)
recordedbreedingby the Brown Booby along Mexico'sPacificcoastsouthonly to
Las IslasTres Marias, Nayarit, thus overlookingthe two coloniesmentionedhere,
althoughGoldman(1951) photographedthe Guerrerocolony.At Las IslasBlancas
(17o40' N, 101o38' W), apparentlynot a breedingsite, we counted200 roosting
Brown Boobies.
Red-footedBooby,S. sula (203; 120/6). Fairlycommon35-100 km offshore
from Michoacansouth over waters deeper than 1500 m; uncommonnorth to
Jalisco.We also saw one brown-plumagedbird roostingat Las IslasBlancas.This
specieswasmore commonlyattractedto the boatthanto feedingflocks,and up to
ten birds often accompaniedus, outmaneuveringthe less agile juvenile Brown
Boobiesin pursuitof flyingfishflushedby the boat. Of the birdswe saw, 99% were
brown-plumagedand at least 80% of these were immatures;we noted only one
white-morphadult and three or four brown adults.27.6-29.5øC.
MagnificentFrigatebird,Fregata magnificens (40; 16/7). Uncommon. Of the
total, we saw 15 on the approach to Manzanilloon 29 April and 16 at sea with
feedingflockson 5 May. Althoughwe notedMagnificentFrigatebirds
as far as 95 km
offshore,most were lessthan 45 km from land. 26.9-28.6øC.
Wilson's Phalarope, Phalaropus tricolor. One flew northwest at 17o53' N,
103ø35' W, on 5 May.
Red-neckedPhalarope,P. Iobatus(2990; 1500/8). Commonto fairlycommon.
We recordedRed-neckedPhalaropesup to 95 km offshorebut mostwere lessthan
45 km from land, and largeconcentrations
occurredwithin 15 km of shore.25.629.5øC.
Red Phalarope,P. fulicaria (487; 190/9). Commonto fairlycommon,occurring
9-95 km offshorebut mostly20-55 km from land. 25.6-29.5øC.
PomarineJaeger,Stercorariuspomarinus(86; 24/9). Common.We saw43 birds
with feedingflocksbut the only kleptoparasitic
chasesnotedwere singlechasesof
Masked and Brown boobies and of a Christmas Shearwater. Immatures were com-
monerthan adultsat feedingflocks,and mostof the 27 adults(alllight-morph)were
headingnorthwest.PomarineJaegersoccurred4-95 km offshore.25.6-29.4øC.
ParasiticJaeger,S. parasiticus(14; 8/6). Uncommon.After eight(includingfour
light-morphadults)on 28 April, thisspecieswas notablyuncommon,with onesand
175
SEABIRD
OBSERVATIONS
twos(mainlyimmatures)
seenmostlyat feedingflocks,wherewe notedtwo chasesof
BlackTerns.All ParasiticJaegerswere 25-95 km offshore.25.6-29.2øC.
Long-tailedJaeger,$. 1ongicaudus
(25; 7/8). Fairlycommon25-95 km offshore,
with one noted9 km from land.The 25 birds(18 at feedingflocks)included17 adults
and 6 immatures,all of the lightmorph. We noted23 chases,all of terns:21 of the
BlackTern, and one each of Arctic and Common terns. 27.5-29.4øC.
None of the adultshadthe longtail streamersof alternateplumage,althoughone
showedslenderstreamersapparentlyhalf-grown;the resthadthe shorterandthicker
streamersof basicplumage.In contrastto this species'highly variablejuvenile
plumage,the immatureswere consistentin appearance,and the availableliterature
(e.g., Cramp and Simmons1983, Harrison1983) suggests
the immatureLong-tailed
Jaegerswe saw were second-summer
birds.Since the immatureplumagesof the
Long-tailedJaegerare poorlyknown,we describethisplumagebriefly.The headand
underbodywerewhitishwith a smudgydarkcap suggesting
an adult,a buffy-yellow
washon the sidesof the neck,andan obviousdarkchestband.The underwings
were
darkwith a distinctbutnarrowwhiteflashalongthe basesof the primariesand strong
pale barringon the coverts.The upperpartsresembledthoseof the adults,but the
brownish-gray
upperwingcovertsdidnot contrastas stronglywith the darkremiges,
and the upper tail covertswere barred whitish. We were unableto discernany
projectingtail streamers.
FrankliftsGull,Larus pipixcan (3; 1/2). Rare. We sawonlythree, all in alternate
plumage,9-65 km offshore,althoughthisspecies
wasfairlycommonin coastalbays
and harbors. 27.5-29.2øC.
Sabine'sGull,L. sabini(112; 38/8). Fairlycommonto common4-75 km (mostly
10-55 km) offshore.Mostwere seenin transit,but onesand twosoccasionally
associated
with feedingflocksand 11 were with one flock.In thisflockwe observed
up to two Sabine'sGullsat a time (andat leastfive individuals
in all)makingseven
kleptoparasiticchasesof Black Terns; at least two chaseswere successful.25.829.4øC.
Fewerthan10%of theSabine's
Gullswe sawwerefullyalternate-plumaged
adults,
the restapparentlybeingfirst-yearbirdswith partialhoodsor mostlydark hoods
fleckedwhitish.Someof thesemayhavebeenadultsthathadnot completed
their
prealternate
molt,althoughthisreportedly
occursbeforenorthwardmigration(Grant
1986). Alternatively,
somemay havebeenadultsthat winteredlocallyand had not
yetcompleted
theirmolt.AlthoughtheA.O.U. (1983)described
thewinterrangeof
Sabine'sGull as "Panamasouthto centralChile,"the specieswinters,at leastin
someyears,northto watersoff westernMexico,as indicatedby the following:
HowellandP. PylesawfouradultsbetweenCaboSanLucas,BajaCaliforniaSur,and
PuertoVallarta,Jalisco,on 17 December1983; J. C. Arvin(pers.comm.,Figure4)
saw200-250 Sabine'sGulls,mainlybasioplumaged
adults,overthe GordaBanks
approximately10-20 km southof San Josedel Cabo, Baja CaliforniaSur, on 2
March1984 and28 February1985. Grant(1986) published
photographs,
takenoff
El Salvadorin March1980, of adultandfirst-year
Sabine'sGullsshowingextensive
flightfeathermolt.
CommonTern,Sterna hitundo(30; 9/6). Uncommonto fairlycommon2-80 km
(mostlylessthan45 kin)offshore.The majoritywerewithfeedingflocks,andthe 30
birdsincluded15 adultsand 12 first-yearbirds.27.1-29.5øC.
ArcticTern, $. paradisaea(74; 26/6). Fairlycommon30-95 km offshore,with
mostat least55 km fromshoreoverwatersmorethan 1800 m deep.The majority
werewithfeedingflocks,andthe74 birdsincluded
62 adultsandtwofirst-year
birds.
26.9-29.4øC.
176
SEABIRD OBSERVATIONS
Jehl (1974) stated, "The northward migration route of the Arctic Tern . . is
unknown,"and certainlyfor the Pacificthe literatureon this subjectappearsto be
rather vague. Our observationsindicate that some Arctic Terns move north off
westernMexicoin springbut that they occurfar offshore.
Least Tern, S. antillarum (41; 23/6). Uncommon to fairly common 2-30 km
(mostlylessthan 18 km) offshoreover waterslessthan 1500 m deep. Like other
terns.thisspeciesusuallyassociated
with feedingflocks.We notedonlyone first-year
bird. 27.5-29.5øC.
BridledTern S. anaethetus(325: 260/5). Locallycommon.Within35 km (mostly
within18 kin)of landoverwaterslessthan 1200 m deepoff Zihuatanejo
we saw60
adultBridledTernson 3 May and 260 adultson 4 May. Uncommonawayfrom this
area, with singleadultsoff Jaliscoand Colima, and three adultsabout 55 km off
Guerrero on 1 May. 27.6-29.2øC.
Howell et al. (1990) consideredit "likelythat Zihuatanejowas, and may stillbe, the
siteof a breedingcolony"(of BridledTerns).K. L. Garrett(pers.comm.)recently
foundat the LosAngelesCountyMuseuma specimenof a BridledTernmislabeled
as
a Sooty Tern. This bird was collectedat "White Friars, Mexico" (17028' N, 101031 '
W) on 7 May 1939. and the labelreads"nestingfrom surf to summit."From the
coordinates,
WhiteFriarsappearsto be synonymous
with LasRocasPotosi(thereare
no rocks4 nauticalmilessouthof LasRocasPotosi).On 3 May 1992 we circledLas
RocasPotosi,1330-1410, andsawa fewBridledTernsaroundthe rocksbutno sign
of nesting:year-to-year
variationin nestingchronology,
El Nifio, or simplytime of
day couldaccountfor the apparentabsenceof birdsat the rocks.
A previouslyunreportednestingsite for thisspecies(andits northernmostin the
easternPacific)is lslaIsabel,Nayarit,wherewe estimated20-30 pairson 9 May
Figure4. Sabine'sGulls10 km southof SanJosedelCabo,BajaCalifornia,Sur,28
February1985. This speciesoccursoff Mexicoduringsome winters perhaps
regularly.
Photo by John C. Arvin
177
SEABIRD OBSERVATIONS
1992, including
birdsdisplaying,
copulating,andenteringandleavingcrevicesin the
rockystacksoff the easternsideof the island.
SootyTern,S. fuscata(8; 5/3). Uncommon.We sawonlyfouradultandfourfirstyearSootyTerns,allwithfeedingflocks40-55 km offshoreoverwaters1300-3600
m deep. 27.9-29.2øC.
BlackTern, Chlidoniasniger (2061; 522/9). Commonto fairlycommon.We saw
BlackTernsup to 80 km offshore,
butmostwerewithin35 km of landoverwaters
lessthan 2000 m deep. BlackTernswere characteristic
of inshorewaters,where
theywerea dominantcomponent
of feedingflocks.In feedingflocksoff Guerrero,
80-90% of the BlackTernswere immatures,andonly 10% were alternate-plumaged
adults;off Jaliscoand Colima, 60-90% were alternate-plumaged
adults. 26.029.5øC.
BrownNoddy,Anousstolidus(41; 25/3). Fairlycommonoff Zihuatanejo
within
35 km of shoreoverwaterslessthan 1000 m deep.We sawonlyone noddyaway
from this area. 27.5-29.4øC.
BrownNoddies
havenotbeenreportedbreeding
alongtheMexicancoastsouthof
Las IslasTres Marietas,Nayarit (A.O.U. 1983), but they may nest at Las Rocas
Potosi,wherewe counted200-250 adultson 3 May.
Brewer'sBlackbird,Euphaguscyanocephalus.A male circledthe boat at 19o36'
N, 106ø05' W on 28 April.
Brown-headedCowbird, Molothrus ater. A male circledthe boat at 17o27' N,
103ø20'Won 30 April, and a femalecircledthe boat at 19ø15'N, 105o28' W on 6
May.
SpeciesNot Seen
Pterodromapetrelswere notableby their absence.At leastat thisseason
few or none apparentlyoccurwithin 75-95 km of shorein the region,
despitethe deep watersof the MiddleAmericanTrench. Pitman (1986)
mapped severalspeciesof Pterodroma as occurringoff westernMexico
(mostlybetweenMay and October;R. L. Pitmanpers. comm.),but, given
the scaleand formatemployed,none may havebeenwithin 95 km of the
mainland.
FeedingFlocks
Feedingflocks,predominatelyof shearwaters,
boobies,and terns,were
conspicuous
duringour cruise.Up to 16 jaegers(three species)occurred,
but only Long-tailedJaegersmade regularkleptoparasiticchases.Small
numbersof storm-petrels(particularlythe Black Storm-Petrel)associated
with 16 of the flocksbutrarelywerenotedfeeding.Seventypercentof one
feedingflock, however,consistedof Blackand Galapagosstorm-petrels;
thiswasthe onlyfeedingflockassociated
with BottlenoseDolphins.
Seventeenof the 34 flocksobserved,
including
the mostdiverse(average
10.8 speciesper flock),occurredbetweenthe inneredgeof the trenchand
25 km of shoreover depthsof 1000-4500 m; 16 of theseflockswere
associatedwith dolphins.The commonerspeciesat theseflockswere the
Pink-footed,Wedge-tailed,and Townsend'sshearwaters,Brown Booby,
PomarineJaeger,and BlackTern, sometimeswith with smallernumbersof
Christmas and Audubon's
bies.
178
shearwaters and Masked and Red-looted
boo-
SEABIRD
OBSERVATIONS
Five feedingflocks(average7.2 speciesper flock)occurredover the
trenchoverwater4200-5200 m deep;fourof theseflockswereassociated
with dolphins.The Pink-lootedand Townsend's
shearwaters
and Brown
Boobywere the commonerspeciesin theseflocks.
The remaining 12 flocks (average3.8 speciesper flock) were over
inshorewaters lessthan 1000 m deep. Only one of these flockswas
associated
with dolphins,but nine otherswere overschoolingfish apparentlyfeedingat the surface.Terns,particularly
the Black,dominatedthese
flocks, with smaller and/or more local numbersof the Brown Booby,
Sabine'sGull, Leastand Bridledterns, and Brown Noddy.
Severalspecies,notablyternsandshearwaters,
weredetectedcommonly
at feedingflocksbut rarely(or not at all in the caseof SootyTern)during
censuses in transit.
Au and Pitman (1986) discussed
seabirdinteractionswith dolphinsand
tuna in the easterntropicalPacific.Both seabirdsand dolphinscommonly
Table 2 Compositionof SeabirdFeedingFlocksAssociatedwith
Schoolsof Spottedand Mixed Spottedand SpinnerDolphinsin the
EasternTropicalPacific
Au and Pitman (1986) a
n
LaysanAlbatross
Pterodrorna petrels
Pink-looted Shearwater
Wedge-tailedShearwater
%
0
99
n
%
1
0
0.6
•c
277
0.02
6.3
31.0
84
•
237
1.4
222
99
29
0.2
60
1.4
MaskedBooby
•
Brown Booby
Red-lootedBooby+ Boobysp.
371
1130
5583
2.2
6.8
33.4
44
2632
85
1.0
61.1
1.9
Tropicbirdspp.
Frigatebirdspp.
Red Phalarope
Jaegerspp.
1
617
0
2168
0
19
55
62
0.4
1.3
1.4
1
25
0.02
0.6
Townsend's Shearwater
Other shearwaters
Storm-Petrelspp.
5184
This cruiseb
Franklin's Gull
Sabine's Gull
Sooty Tern
0.01
3.7
13.0
0
0
955
Black Tern
5.7
9
Noddyterns
175
1.0
Other terns
161
1.0
Total
16710
9
613
8
90
1.9
5.0
2.3
0.2
14.0
0.2
2.0
4386
a134 flocks,January-March.
b20 flocks,April-May.
CSpecies
possiblyincludedin "other"categoriesby Au and Pitman(1986).
dlncludes
Blue-lootedBoobyin Au andPitman(1986).
179
SEABIRD OBSERVATIONS
associatewith yellowfintuna (Thunnus albacares)and feed on smallerfish
and other prey forcednear the surfaceby the tuna, a relationshipmost
pronouncedin the northeasterntropicalPacific.
Our observations
agreewith Au and Pitman's(1986) in that the Spotted
(Stenella attenuata) and Spinner (S. Iongirostris)were the common
dolphin speciesassociatedwith the flocks. Bird speciescompositionof
feeding flocks associatedwith dolphins, however, differed strikingly
(Table2). Some of thesedifferencescan be readilyexplainedby our cruise
being closerinshore:compare proportionsof the more coastalBrown
Boobyand BlackTern with thoseof the more pelagicRed-lootedBooby
and SootyTern. The seasonalsoexplainssomeof the differences:
Au and
Pitman'sflockswere observedfrom Januarythough March when several
northboundtransientswe notedin April and May (e.g., Sabine'sGull,Arctic
Tern) wouldhavebeen rare or absentand when manywinteringPomarine
Jaegerswerestillpresent.Bothseasonalandinshoreeffectsprobablyapply
in some cases, as with the Pink-looted Shearwater and Black Tern, and
year-to-yearvariationand E1Nifio may be otherfactors.
SUMMARY
From28 April to 6 May 1992 we cruisedalongthe coastof southwestern
MexicofromJaliscoto Guerrero,observing
seabirds.
Thisareaisimportant
for non-breeding
Townsend'sShearwaters.We foundChristmasShearwatersto be fairlycommon.We notedLong-tailedJaegersandArcticTernsas
fairly commonspringmigrants.Feedingflocksof up to 15 specieswere
conspicuous,
themostdiverseflocksbeingin a belt25-55 km offshoreover
waters 1100-3250 m deep, inshoreof the MiddleAmericanTrench;most
were associated
with SpottedDolphins.We report previouslyunpublished
coastalbreedingcoloniesof the Brown Booby,Red-billedTropicbird,and
Bridled Tern.
ACKNOWLEDGMENTS
We thank the Kelton Foundationfor making possibleour cruise off western
Mexico. To captainRichardKelton and fellow crew membersBob Chappel, Fred
Geller, and Mary Nichollswe extend our thanksfor making the trip run smoothly.
RobertL. Pitmanand Larry B. Spear kindlymadeavailablecertainunpublisheddata.
KimballL. Garrett provideddata on specimenshousedat the Los AngelesCounty
Museum,and Jim Cunninghamand BetseyCutlerallowedaccessto specimensat the
CaliforniaAcademyof Sciences.The Secretariade DesarrolloUrbano y Ecolog•a,
throughthe officesof Dra. Gracielade la GarzaGarcia,granteduspermissionto visit
lsla Isabel.David G. Ainley and Larry B. Spear provideda thermometerand advice
on data collection;they and J. R. Jehl, Jr., madehelpfulcommentson the manuscript.This is contribution578 of Point ReyesBird Observatory.
LITERATURE
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180
SEABIRD
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181